Why do most substances have to be assisted through the plasma membrane?

Answer 1

The plasma membrane is made from tightlypack phospholipids. The plasma membrane prevents polar molecules and large molecules from diffusing freely. Fatty (lipophilic) molecules can easily pass through. since cells often need water soluble materials such as water and sugars, transporters and pores need to be made out of proteins to let those molecules through.

One of the most important pumps is the Na+/K+ ATPase pump which maintains gradients of sodium and potassium across the cell membrane